Recently, electrophotographic or inkjet color image forming apparatuses represented by a color printer and color copying machine require higher quality of an output image.
However, the color of the image obtained by the color image forming apparatus varies when each part of the apparatus varies upon a change in environment or long-time use. Especially, in the electrophotographic color image forming apparatus, the color may vary upon even small environmental variations, thereby losing color balance. Hence, the electrophotographic color image forming apparatus has a means for stably reproducing the color and color tonality. For example, the color image forming apparatus comprises, for toner of each color, process conditions such as several exposure amounts and bias for development in accordance with different absolute humidities, and a tonality correction means such as a look-up table (LUT). The color image forming apparatus selects process conditions optimal for the environment and the optimal value of tonality correction on the basis of an absolute humidity measured by a temperature/humidity sensor. In order to obtain a constant color and color tonality even upon variations in each part of the apparatus, the following density control is performed (see Japanese Patent Laid-Open No. 7-055703, and Japanese Patent No. 3430702). First, a toner patch for detecting density is formed on an intermediate transfer material, photosensitive drum, or the like with each of single-color toners. The density of the unfixed single-color toner patch is detected by an unfixed toner density detection sensor (to be referred to as a density sensor hereinafter). Then, feedback control of the tonality correction means such as the process conditions (e.g., the exposure amount and the bias for development) and the LUT is done on the basis of the detection result.
In density control using the above density sensor, a patch (test image) is formed on an intermediate transfer material, photosensitive drum, or the like, and the density is detected. A change in the color balance of an image subsequently transferred and fixed onto a transfer material is not controlled. The color balance also changes depending on the transfer efficiency of transferring a toner image onto a transfer material and the heating and press for fixing. Such change cannot be dealt with by the density control using the above density sensor.
Also, since only the single-color patch is formed in this density control, a change in the color balance of the image that is caused by mixing of the plurality of color toners is not controlled.
To solve this problem, the following color image forming apparatus has been proposed (for example, see Japanese Patent Laid-Open No. 2003-084532), which comprises a sensor (to be referred to as a color sensor hereinafter) for detecting the chromaticity and density of the patch on a transfer material such that the mixture rate of cyan (C), magenta (M), and yellow (Y) for forming an achromatic gray scale image of a process gray patch can be output by forming a gray patch of black (K) and the process gray patch of C, M, and Y on the transfer material, and comparing the gray patch of K with the process gray patch of C, M, and Y as a reference after fixing the toner.
In this color image forming apparatus, the detection result is fed back to, e.g., a color matching table for converting the exposure amount and process conditions of the image forming section and the RGB signal of the image processor into the color reproduction range of the color image forming apparatus, a color separation table for converting the RGB signal into a CMYK signal, and a calibration table for correcting characteristics of density to tonality. With this operation, the density or chromaticity control of the final output image on the transfer material can be performed. The output image formed by the color image forming apparatus can be detected by an external image reading apparatus or a colorimeter/densitometer to perform the same control. In addition to this, the advantage of this scheme is that the control can be completely performed in the image forming apparatus. For example, this color sensor includes three or more types of filters having different spectral transmittances of, e.g., red (R), green (G), and blue (B) on a light-receiving device by using three or more types of light sources having emission spectra of, e.g., red (R), green (G), and blue (B) as a light-emitting device, or using a light source for emitting white (W) light as the light-emitting device. In this arrangement, three or more types of outputs such as R, G, and B outputs can be obtained.
In the inkjet color image forming apparatus, the color balance also changes due to aging or the environmental difference of an ink discharge amount, or the individual difference of ink cartridges. The characteristics of density to tonality cannot be held constant. To solve this problem, in some cases, the image forming apparatus detects the density or chromaticity of the patch on the transfer material to perform the density or chromaticity control, by using an inkjet head in place of the color sensor.
However, since only one color sensor is mounted for each conventional electrophotographic color image forming apparatus, the patch must be formed at a detectable position limited depending on a sensor position such as the center of the transfer material in a direction (to be referred to as a scan direction (first direction) hereinafter) substantially perpendicular to (crossing) the moving (convey) direction of the transfer material.
Alternatively, in the electrophotographic color image forming apparatus, the density and chromaticity vary even with a patch having the same signal in the scan direction, thus posing a problem. This is caused by small variations in transfer characteristics of toner at the center and side of the transfer material, and small variations in fixing characteristics of a fixing unit which fixes the toner onto the transfer material by heating and pressing, at the center and side of the transfer material.
Therefore, the conventional color image forming apparatus including the color sensor can improve the color reproducibility, but cannot follow the density and chromaticity variations on the single transfer material.
Also, since the density and chromaticity can be detected only by using the patches formed at the determined position such as the center of the transfer material, the number of patches to be formed on the single transfer material is limited. Also, in order to form the patches for controlling the density or chromaticity, the color image forming apparatus must form patches on a plurality of transfer material in one control process, or reduce the number of patches to form a maximum number of patches on the single transfer material with lower precision.
In the inkjet color image forming apparatus which adopts a scheme for exchanging the inkjet head for the color sensor, since the color sensor moves in the scan direction as the inkjet head, the electrophotographic color image forming apparatus has no problem. However, a user must exchange the inkjet head for the color sensor, resulting in a cumbersome operation.
In addition to this, while the electrophotographic color image forming apparatus may include a movable color sensor as the inkjet color image forming apparatus, in this scheme, the transfer material must be stopped outside the fixing unit when moving the color sensor in the scan direction. In order to perform such processing, the color sensor must be separated from the fixing unit by at least the length of the longest transfer material. Hence, this scheme cannot cope with a color image forming apparatus in which the fixing unit is near the discharge section.